An ignition coil for an internal combustion engine has a coil portion disposed at a middle portion, a control portion disposed at an upper end portion and a high voltage tower portion disposed at a lower end portion in a longitudinal direction thereof. The control portion includes an igniter for interrupting a current through a primary coil in the coil portion to generate a large voltage in a secondary coil in the coil portion. The large voltage is transmitted via the high voltage tower portion having cables and the like to a spark plug. Some ignition coil has a structure directly connected to the spark plug without cables and the like.
As shown in FIG. 7, a conventional ignition coil 100 as disclosed in JP-2003-28039-A has a primary coil 101, a secondary coil 102 and a connector portion 103 at a bottom side of the primary and secondary coils 101, 102. A spark plug 105 has an electrical insulating connector portion 106 at a top end portion thereof. The connector portion 103 of the ignition coil 100 and the connector portion 106 of the spark plug 105 are connected to each other to be enclosed in a plug pipe 107. The ignition device disclosed in JP-2003-28039-A, however, has a complicated structure having relatively many components and requiring much assembling works.
As shown in FIG. 6, a configuration can be thought to solve the above-described issue to dispose components of the ignition coil 85 and the spark plug 86 in a cylinder-shaped insulator 88. In an ignition device shown in FIG. 6, a center core 91, a secondary spool 92 and a secondary coil 93 are enclosed in a center bore formed in a large diameter portion 89a, an upper half portion of the insulator 88. A primary coil 94 is wound on an outer circumferential face of the large diameter portion 89a. A center electrode 96 is disposed in a center bore formed in a small diameter portion 89b, a lower half portion of the insulator 88. Further, a thin cylinder-shaped case 97 is disposed around the insulator 88. The case 97 is made of magnetic material. Still further, a thin cylinder-shaped tube 98 is disposed around the case 97 to form a clearance 99 therebetween. The tube 98 is made of electrical insulating material.
The tube 98 serves as a partition to prevent lubricating oil in an intake/exhaust valve assembly (not shown) disposed by a side of the coil portion 85 from entering in the coil portion 85 and the spark plug 96. The intake/exhaust valve assembly includes an intake valve, an exhaust valve and valve moving mechanism for the intake and exhaust valves. The valve moving mechanism includes a cam and a rocker arm oscillated by the cam to move the intake valve and the exhaust valve. The lubricating oil is supplied to a contact portion between the cam and the rocker arm. The tube 98 prevents the lubricating oil from flowing along the ignition device to the plug portion 96. If the cam is in a direct contact with the intake and exhaust valves and the like not via the rocker arm, the lubricating oil is supplied to a contact portion between the cam and the intake and exhaust valves and the like.
The ignition device shown in FIG. 6, however, still has an issue that an outer diameter of the ignition coil 85 is large.
Firstly, the case 97 and the tube 98 make the outer diameter of the ignition coil 85 large. The case 97 made of magnetic material is disposed radially outside the center core 91, the secondary winding 93, the primary winding 94 and so on and serves a peripheral core of the ignition coil 85 that is indispensable as a component of the ignition coil 85. The tube made of electrical insulating material is inserted in a top end portion of the plug hole and is indispensable to prevent the lubricating oil from entering in the spark plug. The case 97 and the tube 98 separately formed from the case 97, however, make the outer diameter of the ignition coil 85 large.
Secondly, a cylindrically shaped clearance 99 between the case 97 and the tube 98 makes the outer diameter of the ignition coil 85 large. The clearance 99 is necessary for installing the ignition device in the plug hole of the internal combustion engine.
Currently, internal combustion engines are manufactured in small dimensions, so that the ignition plug and the spark plug are disposed close to the valve moving mechanism having a complex structure on a relatively small area on the cylinder head. Thus, it is required to reduce the outer diameters of the ignition plug and the spark plug. Especially, it is required to reduce a thickness of the tube 98 and that of the clearance 99, which are provided for preventing lubricating oil from entering in the ignition coil and the spark plug and have no principal function of the ignition coil and the spark plug.